Zinc-copper exchange

Finally, zinc-copper exchange by treatment of FG RZnI witli Me2CufCN)Li2 provides copper species thal add smootlily lo various alkynes and which can also he used lo perforcn cyclizalion reactions fScheme 2.48) [98]. 

Ziegeler-Natta conditions 304 zig zag chromophores 217 zinc-copper exchange 275 zinc-copper organometallics 397 zinc-copper reagents 396, 397 reactions with acid chlorides 398 zinc-copper species RCu(CN)ZnX 394 zinc enolates 20 zinc metal, activated 291 zinc organometallics 278, 389 zinc salts 9... 

The properties of hydrated titanium dioxide as an ion-exchange (qv) medium have been widely studied (51—55). Separations include those of alkaH and alkaline-earth metals, zinc, copper, cobalt, cesium, strontium, and barium. The use of hydrated titanium dioxide to separate uranium from seawater and also for the treatment of radioactive wastes from nuclear-reactor installations has been proposed (56). 

Phinney, J. T. and Bruland, K. W. (1997). Trace metal exchange in solution by the fungicides Ziram and Maneb (dithiocarbamates) and subsequent uptake of lipophilic organic zinc, copper and lead complexes into phytoplankton cells, Environ. Toxicol. Chem., 16, 2046-2053. 

Electrophiles, which lead to high yields, are methyl iodide, trialkyltin- and trialkylsUyl chlorides, diphenylphosphinyl chloride, acid chlorides, aldehydes and carbon dioxide. Remarkably, though highly acidic ketones are formed on acylation, no deprotonation or racemization by excess of carbanionic species occurs. Other alkyl halides than methyl iodide react very sluggishly with low yields. Benzylic and aUylic halides lead to partial racemization, presumably due to single-electron transfer (SET) in the alkylation step. As very recently found by Papillon and Taylor, racemization of 42 can be suppressed by copper-zinc-lithium exchange before alkylation to 43 via the Knochel cuprates (equation 7) °. 

Alkynyl iodides and bromides react smoothly with various zinc-copper organometaUics at — 60 "C leading to polyfunctional aUcynes. lodoalkynes, such as 296, react at very low temperature, but lead in some cases to copper acetylides as by-products (1/Cu exchange reaction). 1-Bromoalkynes are the preferred substrates. Corey and Helel have prepared a key intennediate 297 of the side chain of Cicaprost by reacting the chiral zinc... 

Here, the adsorption of valine on different cation-exchanged montmorillonites is described (Nagy and Konya 2004). A discussion of the kinds of interactions that are possible in the ternary system of montmorillonite/valine/metal ions will be presented, and a description how the metal ions can affect these interactions. The interlayer cations (calcium, zinc, copper ions) were chosen on the basis of the stability constants of their complexes with valine. The adsorption of valine on montmorillonite is interpreted using a surface-complexation model. 

Anion exchange was achieved by mixing 5 g of ZCA HDS with 500 mL of 0.1 M sodium stearate solution in chloroform for 24 h at room temperature with frequent shaking. The supernatant was decanted and replaced by a fresh solution for another 24 h with this process repeated for the third time. The exchanged HDS product, zinc copper hydroxy stearate (ZCHS) was recovered by filtration, washed with 500 mL of methanol, followed by 500 mL of acetone and plentiful amounts of water before drying at 100°C overnight to drive off residual solvents. 

Cross-coupling between functionalized zinc-copper reagents and l-iodoalkynes or l-bromoalkynes is very fast [83]. This smooth cross-coupling occurs at low temperatures (—55 "C) and offers hi stereoselectivity in reactions with chiral secondary organozinc-copper reagents such as 108 (obtained by a hydroboration/ boron-zinc exchange sequence), producing the alhyne 109 in 42% overall yield (Scheme 2.40) [73]. 

Ion exchange can be useful for heavy metal removal, particularly for nickel, zinc, copper, or chrome, where the metals can be recovered from the regenerating solution and recycled to the process or sold. Ion exchange has also been applied to treatment of streams containing complexing agents or their compounds, that would interfere with a precipitation process. 

When aqueous ammonia is added to aqua complexes of d block elements such as those of nickel, copper and zinc, ligand exchange takes place and a solution of the ammine complex is formed. 

Much emphasis has been placed in recent years on the use of chelates in supplying plants with iron, zinc, copper and manganese. Since chelation reactions are believed to be involved in the reaction of humus with these metal ions it is appropriate to consider what is meant by chelation and to compare the status of metals held in this manner with that of those held in the organic matter exchange complex. Incidentally, the formation of soluble metal—organic complexes has long been known although emphasized only comparatively recently (Horner et al., 1934). 

In addition to the extensive use of Uthium and magnesium, halogen-metal exchange of haloindole substrates has been investigated with zinc, copper, boron, and tin. 

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